There’s no good answer for this, since different people progress at different rates. A “minimum dose” of practice is virtually impossible to predict.
To give you some context, I practice pretty much daily. There are times when I’ll grind drills and phrases for hours, there are other times where I only pick up my bachi for 10 or 20 minutes (enough to play through a few songs). I also have dedicated practice sessions with my teacher three times a month for 90 minutes at a time.
Prior to Covid-19, I met with my tsugaru teacher twice a month for 30 minutes at a time.
If you’re practicing twice a week, I don’t think you should worry too much about learning quickly. Focus instead on incremental progress. A new phrase. A cleaner strike.
If your worry about getting better fast, you’ll burn out.
Just play and play mindfully.